Highly clean plastic film or sheet and process for its production

ABSTRACT

A plastic film or sheet, wherein upon immersing a test piece of the plastic film or sheet in ultrapure water, extracting pure water from near the surface of the test piece and evaluating the cleanliness of a resulting bag based on the concentration (number) of fine particles 0.3 μm or greater in size dispersed in the extracted ultrapure water, the measured concentration is no greater than 5 per ml. The process comprises steps of immersing and running a plastic film or sheet (1) in ultrapure water (4) in a clean room (16), and further spraying and washing the film or sheet (1) with ultrapure water after the film or sheet (1) is drawn out, followed by drying, destaticization, cutting and heat sealing.

TECHNICAL FIELD

The present invention relates to a highly clean plastic film or sheet,and especially it relates to a plastic bag with particularly highcleanliness, which is used in production of semiconductors, precisioninstruments, electronic devices and the like, for medical and biorelatedappliances, or for wrapping garment/gown, gloves, etc. used in cleanrooms.

BACKGROUND ART

Semiconductor devices and parts are extremely hindered by dirt, dust andother contamination. Such devices and parts are therefore produced inclean rooms, while the garments, gloves and other operating utensilsused must all be clean.

Clean rooms are controlled so as to maintain a prescribed degree ofcleanliness. When clean parts, utensils, garments and the like forproduction of semiconductors are placed in bags to be brought into cleanrooms, it is essential to use clean bags which are, naturally, free ofdust inside, but which are also free of dust on both the inner and outersurfaces, since dust clinging to the outer surface will contaminate theclean room.

Known methods for cleaning of bags include washing in ultrapure water,and cleaning of films or sheets prior to manufacturing of the bagsinside clean rooms. Another method involves coating both the front andback sides of an inner film or sheet for wrapping with a peelable outerfilm or sheet to prevent contamination of the inner film or sheet, andpeeling off the outer film or sheet at the time of use and using theinner film or sheet to wrap a given article to be wrapped (JapaneseUnexamined Patent Publication No. 6-285944).

In the method involving coating of the film or sheet with an outerlayer, the bag must be wrapped after peeling off of each outer layer,while waste of the outer layer is also not economical.

Although cleaning of films and sheets by washing with ultrapure water iscommonly known, almost no specific methods have been published. Suitableultrapure water must be prepared for high cleaning of films and sheets,and the washing and drying methods must be specially modified. As aresult, it has not been possible to achieve sufficient cleanliness ofconventional plastic bags even by washing with ultrapure water.

In general, the cleanliness of films, sheets and bags is expressedrelative to the clean room, such as in terms of causing no contaminationof the clean room even when, for example, a class 10 clean room(according to Fed. Std. 209D. Same hereunder) is used, and thecleanliness of the sheets or bags themselves is never expressedquantitatively. Therefore, the expression of cleanliness is impreciseand subjectively influenced.

Measurement of fine particles on the surfaces of films or sheets can beaccomplished, for example, with an optical microscope, but generallyonly sizes of 5 μm or greater may be detected, while fine particles withextremely small sizes of under 1 μm are almost impossible to detect. Inaddition, the very narrow visual field of the microscope does not alloweasy measurement of fine particles in sections having certain areas.Detection of fine particles is also difficult in sections where thesurface is bent or irregular.

The present invention provides a highly cleaned plastic film or sheetand a process for its production, and more preferably it provides aplastic film or sheet with high cleanliness through the successiveprocess from molding of the film or sheet until its cleaning andformation into a bag, and particularly a process for manufacturingplastic bags.

DISCLOSURE OF THE INVENTION

In the process employed according to the invention, the degree of finecontaminating particles adhering to plastic films and sheets areevaluated, and the cleanliness of the plastic films or sheets isexpressed in those numerical terms. Also, as a result of a modifiedwashing process for the plastic films or sheets using a prescribedamount of ultrapure water, it is possible to produce plastic films andsheets with a high degree of cleanliness which has not been obtainableaccording to the prior art as compared in terms of the aforementionednumerical values.

In other words, the present invention provides a highly clean plasticfilm or sheet, wherein upon immersing a test piece of the plastic filmor sheet in ultrapure water, extracting pure water from near the surfaceof the test piece and measuring the concentration (number) of fineparticles 0.3 μm or greater in size dispersed in the extracted ultrapurewater, the concentration is no greater than 1000 per ml, preferably nogreater than 100 per ml, ideally no greater than 10 per ml andespecially no greater than 5 per ml on both sides of the plastic film orsheet. Such highly clean plastic films and sheets have not hitherto beenprovided.

According to the present invention there is likewise provided a processfor high cleaning of a plastic film or sheet, which is characterized inthat

(A) a plastic film or sheet is highly cleaned in a clean room, and

(B) said high-cleaning process comprises the steps of:

i) immersing the plastic film or sheet in ultrapure water for washing,

ii) exposing both sides of the plastic film or sheet drawn from theultrapure water to spraying with ultrapure water for forcible washing,

iii) removing the water from the plastic film or sheet, and

iv) destaticizing the plastic film or sheet.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual drawing showing the process for measuring thefine particle concentration (surface-adhering amount) for a plastic filmor sheet sample.

FIG. 2 is a sketch of a cleaning apparatus for high-cleaning of aplastic film or sheet according to the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention relates to a high cleanliness plastic film orsheet wherein upon immersing a test piece of the plastic film or sheetin ultrapure water, extracting pure water from near the surface of thetest piece and measuring the concentration (number) of fine particles0.3 μm or greater in size released and dispersed in the ultrapure water,the concentration is no greater than 1000 per ml, preferably, no greaterthan 100 per ml, most preferably no greater than 10 per ml, particularlyno greater than 5.0 per ml on both sides of the plastic film or sheet(the inner and outer sides in the case of a bag).

The material of the plastic film or sheet used may be any commonly usedone such as polyethylene, polypropylene, nylon, polyester,ethylene-vinyl alcohol copolymer, polyvinylidene chloride, and it is notparticularly restricted so long as it can be formed into a film orsheet. These may also be used either alone or in laminated layers.Particularly preferred are films and sheets with electrically conductivecoated surfaces, for a surface electric resistance of 10⁹ Ω/□ or less toprevent static electricity. It is thus possible to prevent electrostaticadhesion of fine particles. An ideal multilayered film is described inJapanese Unexamined Patent Publication No. 62-94548. The thickness ofthe sheet or bag is not particularly restricted, and may normally be inthe range of 30-150 μm. Plastic film or sheet products with a thicknessof 1/100 inches (0.254 mm) or less are usually referred to as filmswhile thicker products are referred to as sheets, and both are intendedto be included.

The plastic film or sheet of the invention has fine particles of 0.3 μmor greater among the contamination on both sides (the inner and outersides in the case of a bag) in a number of no greater than 1000 per ml,preferably no greater than 100 per ml, more preferably no greater than10 per ml and especially no greater than 5 per ml, as evaluated by themeasuring method described later. The contamination includes dirt, dust,particles, and the like, and most of it consists of fine particles about0.1-10 μm in size. The number of fine particles is difficult to bemeasured directly by optical microscope or other microscopes, andtherefore the following method was used according to the invention. Thismethod has already been established and is described on pages 185-186 ofthe Proceedings of the 11th Air Cleaning and Contamination ControlResearch Convention (sponsored by JAPAN AIR CLEANING ASSOCIATION) heldfrom Apr. 21-22, 1992 ("Evaluation of Surface Contamination by MeasuringParticles Transferred to Pure Water"--Shuji Fujii, et al.) and on pages145-148 of the Proceedings of the 12th Air Cleaning and ContaminationControl Research Convention (same sponsor) held from Apr. 20-21, 1993("Evaluation of Surface Contamination by Measuring Concentration ofParticles Transferred to Pure Water"--same authors).

The method will first be summarized with reference to FIG. 1. Thespecific measuring conditions will be described later.

A test piece 22 to be measured is immersed in a tank 20 holdingultrapure water 21. The fine particles adhering to the surface of thetest piece are thereby released from the test piece as time passes, anddisperse throughout the ultrapure water. At this point, a prescribedamount of ultrapure water is drawn out from the surface of the testpiece by a syringe sampler 35 through a drawing pipe 24, and the fineparticle concentration (number of particles in the sample solution) ismeasured with a light-scattering particle detector.

When the test piece is immersed in the ultrapure water, the amount offine particles released from the test piece surface is proportional tothe concentration of fine particles adhering to the test piece surface,and since the released fine particles continue to disperse in theultrapure water, the concentration of dispersed particles isproportional to the amount of fine particles which have been released.Consequently, the concentration of fine particles in the region ofultrapure water where the fine particles are dispersing near the testpiece surface is roughly proportional to the concentration of fineparticles adhered to the test piece surface. Naturally, since the amountof ultrapure water is sufficiently large compared to the size (surfacearea) of the test piece so as to be considered infinite, the size of thetest piece is assumed not to affect the measurement results. Thedocuments cited above have reported that the degree of cleanliness oftest piece surfaces can be measured by this method and confirmed, whilethe measuring method has since become an established practice in thefield.

The plastic film or sheet of the invention is not restricted, but ispreferably of as high cleanliness as possible.

The method of producing a highly clean plastic bag according to theinvention will now be described. The plastic film or sheet used isprepared by calender working, the blown-film extrusion method, T-diemethod, etc. The blown-film extrusion method is highly productive formaking bags, since films and sheets formed thereby result in tube shapesrequiring only cutting to the length of the bag and sealing of thebottom section. Films or sheets which are not in tube shapes are laidover each other, or one is folded over itself, and the bottom and/orsides are sealed to form a bag.

Since bags formed from tube-shaped films or sheets are not easilycleaned at the inner surfaces, the inner surfaces of the bags arepreferably formed by the blown-film extrusion method in such a mannerthat the degree of cleanliness is within the range of the invention. Asuitable method is one whereby during formation of the blown film orsheet, a highly pure gas, for example high purity nitrogen gas passedthrough a filter to remove fine contaminant particles of 0.2-0.3 μm orgreater, is fed into the tube to blow up the tube. When the tube of afilm or sheet is obtained in this manner, washing of the inner surfaceof the tube is unnecessary, thus allowing streamlining of the washingstep, and it is therefore ideal as a plastic film or sheet for theinvention.

The cleaning of the plastic film or sheet is performed on the outersurface after immersion in the case of a tube-shaped film or sheet, andon both sides in the case of a flat film or sheet. The formation of thefilm or sheet should produce as little contamination in the chamber aspossible, and the outer surface of the film or sheet is also preferablyof higher cleanliness than a normal film or sheet.

During formation of the film or sheet it is impossible to maintain thesurface of the film or sheet at the high level of cleanliness for thepresent invention.

Thus, the plastic film or sheet is washed with ultrapure water in aclean room. The cleanliness of the clean room 16 should be class 100 orhigher, and if possible class 10 or higher. Washing of the plastic filmor sheet with ultrapure water in the clean room, followed by the stepsdescribed later, yielded a highly clean plastic film or sheet for useaccording to the invention.

In the ultrapure water, generally the electrical resistivity is 15-17 MΩ·cm (25° C.) or greater, fine particles of 0.3 μm or greater arepresent in an amount of about 100 per ml or less and SiO₂ in an amountof 10 μg/l or less, but for higher cleaning, preferably the electricalresistivity is about 16 to about 18 M Ω·cm (25° C.), fine particles of0.05 μm or greater are present in an amount of 10 per ml or less andSiO₂ in an amount of 0.1 μg/l or less. The cleanliness must be higherthan the desired cleanliness of the plastic film or sheet.

FIG. 2 shows a method of cleaning a plastic film or sheet with ultrapurewater according to the invention. In FIG. 2, numeral 3 indicates anultrapure water bath which is filled and supplied with ultrapure water4. The plastic film or sheet 1 is wound on a roll 2, the film or sheet 1is fed out from the roll and immersed in the ultrapure water bath 3after being directed around a guide roller 12, and runs through theultrapure water. Numerals 13 and 14 are guide rollers. The ultrapurewater bath is situated on an ultrasonic vibrator 5 to apply ultrasonicwaves to the ultrapure water.

Most of the contaminants adhering to the plastic film or sheet will bereleased by running through the ultrapure water under the ultrasonicwaves, but according to the invention, in order to reliably achieve ahigh level of cleanliness the plastic film or sheet 1 is drawn out ofthe ultrapure water and subjected to spraying with ultrapure water forforcible washing. Numeral 7 indicates spraying nozzles. The sprayingnozzles are provided on both sides of the film or sheet, and thespraying angle is preferably directed at a downward slant with respectto the film or sheet. The spraying conditions for the forcible washingwith ultrapure water were, for example, 17 liters/min for a 600 mm-widefilm, and corresponding conditions are appropriate. The major purpose iswashing off of the contaminated washing water with fresh ultrapure waterin a vertical manner. This also removes contaminants which were notreleased in the ultrapure water bath. The sprayed ultrapure water dropsdown into the ultrapure water bath, and the ultrapure water overflowingfrom the water bath in which fresh ultrapure water is continuouslysupplied is eliminated from a drainage outlet 6. The eliminatedultrapure water is then purified with an ion-exchange resin, reverseosmosis equipment or ultrafiltration membrane filter according to apublicly known method, and is recirculated for use.

The plastic film or sheet which has been cleaned by spraying withultrapure water then enters a drying room 8, where draining and dryingare performed. The drying is accomplished, for example, through waterremoval by blowing clean hot air with an air knife. Numeral 9 indicatesblowing nozzles. Hot air directed downward and slanted with respect tothe film or sheet results in easier drainage and drying. The hot airused is high-purity air with a cleanliness of class 10 or greater, andthough the temperature will depend on the heat resistance of the plasticfilm or sheet, it is generally suitable at between 45-65° C.

The dried plastic film or sheet is then passed through a destaticizer 10for removal of static electricity, and wound around a roll 11. Thedestaticizer used may be a voltage application type, such as a PulserFlow Controller (PFC-20) (trademark) manufactured by Richmond StaticControl Services Inc. A voltage is applied to the surface of the highlycleaned plastic film or sheet to remove the static electricity, to thusprevent electrostatic re-adhesion of the fine particles onto the surfaceof the highly cleaned plastic film or sheet.

The procedure described above is carried out in a clean room 16 with acleanliness of around class 10.

The cleaned plastic film or sheet is then cut if necessary, and whenpreparing a bag, one end of the double layered film or sheet isheat-sealed to form a bag. The cutting and heat-sealing are carried outin a clean room with a cleanliness of class 100 or greater, or evenclass 10 or greater, and the apparatus and utensils used are allcleaned, with care that the bag is not contaminated thereby.

EXAMPLE

The present invention will now be explained in more detail by way of thefollowing example.

A polyethylene used did not contain any additive which breads out aftera product is shaped. A polyethylene tube was formed by a blown-filmextrusion method and wound on a roll. The thickness of the film was 80μm. The lay flat width of the tube or film (width of the film whendoubled over) was 30 cm. The gas fed into the tube was 99.999% nitrogengas from which contaminating fine particles of 0.2 μm or greater hadbeen removed with a filter.

The film wound on the roll was then washed in a washing apparatus suchas shown in FIG. 2. The cleanliness in the clean room 16 in which thewashing apparatus was situated was class 10.

The ultrapure water used had an electrical resistivity of about 16-18 MΩ·cm, and contained ultrafine particles of 0.3 μm or greater in anamount of 0.8 per ml and SiO₂ in an amount of 20 ppb or less. Thespacing between the guide rollers 13 and 14 in FIG. 2 was 0.38 m, andthe film feed rate was 3.6 m/min. The frequency of the ultrasonicvibrator was 38 kHz.

The ultrapure water spraying nozzles 7 were slit-shaped (perpendicularto the plane of FIG. 2) and placed at a downward slant for a water sprayorientation of 45° with respect to the film surface. The water sprayvolume was 17 liters/min at a width of 600 mm.

The hot air for drying had a cleanliness corresponding to class 10, atemperature of 65° C. and an air flow rate of 100 m/min from 4 nozzlesin 2 levels. The blowing direction of the hot air and the nozzle shapewere the same as the spraying nozzles described above.

The plastic film 1 was then passed through a voltage application-typestatic eliminator or destaticizer 10.

The wound up film was cut, heat sealed, and formed into a bag with awidth of 30 cm and a length of 40 cm, in a room with a cleanliness ofclass 10.

The fine particle adhesion state of the bag was then measured. Theapparatus used was the one shown in FIG. 1. A water bath 20 having alength of 21.0 cm and a width of 17.5 cm (not counting the section ofthe overflow bath 25) was filled with ultrapure water to a height of28.0 cm, and allowed to overflow from one side. Numeral 25 indicates anoverflow bath as a reservoir. The ultrapure water is pre-cleaned duringthe measurement. The cleaning is accomplished by passing the ultrapurewater in the overflow bath through a fine particle-collecting filter 29via a circulating pipe 26 and circulating pump 27, feeding it throughanother circulating pipe 30 to the bottom of the water bath, thuscausing it to overflow from the water bath for circulation of thesolution. The fine particle-collecting filter is capable of collectingfine particles of 0.3 μm or greater. Numeral 28 indicates a flow ratemeter.

In this example, the water is circulated for cleaning until the fineparticles with a particle size of 0.3 μm or greater in the water arereduced to 2 per ml or less. Z-propanol (isopropyl alcohol (IPA)) isalso added to the ultrapure water to 5 volume percent to stabilizedispersion of the fine particles.

For the measurement, the background amount of fine particles in theultrapure water is first measured. During the background measurement itis important to take care not to generate air bubbles. The IPA is addedto prevent air bubbles, but there is no point in measurement of the fineparticles while air bubbles are being generated. Circulation of thewater is therefore suspended during this measurement. The measurement isconducted at normal temperature. The ultrapure water is introduced fromthe water bath 20 into a light-dispersing particle detector 32 through adrawing pipe 24 during the measurement. A syringe sampler 35 is used forthis purpose. A 10 ml sample of the ultrapure water is drawn by thesyringe sampler and fed to the light-scattering particle detector, andthe number of fine particles is measured during the feeding. Aftercompletion of this measurement the ultrapure water is fed by the syringesampler to the overflow bath through a feeding pipe 31. The number ofdetected fine particles in the sample solution is calculated by asubmerged particle counter 33. Numeral 36 indicates a control sectionwhich controls the flow rate and drawing rate of the taken sample. Thelight-scattering particle detector is one conforming to the system ofJIS B9925. Detection of particles smaller than 0.3 μm is difficult withlaser light.

The measuring time is about 60 seconds each time, with about 10 secondsfor the water to return from the syringe sampler to the overflow bath.Thus, the measurement is performed about every 70 seconds. Tencontinuous measurements were taken. As a result, the number of fineparticles of 0.3 μm or greater was an average of 1.7 per 10 ml, with astandard deviation of 1.06.

A test piece is then immersed in the bath and measured.

The test piece 22 is gently placed on a platform 23. The test piece isgenerally a square 10-15 cm on each side, or a disk with a diameter ofabout 10-15 cm. Within this range there is virtually no influence on themeasurement by fluctuations in size. In this example there was used arectangle with a length of 15 cm cut from a bag as described above. Theplatform 23 is situated at a height of 3.5 cm from the bottom of thebath. The platform used was a thin plate with multiple pores (diameter:0.5 mm, center spacing of pores: 0.8 mm).

The test piece was placed at the center of the platform and measurementwas started after 2 minutes. The tip of the drawing pipe 24 was broughtnear the surface of the test piece (within 1 mm), and 10 ml of solutionwas drawn out. The tip of the drawing pipe preferably has a slantedopening with an angle of 45° as shown in the drawing. This prevents thetip of the pipe from clinging onto the test piece and also provides agreater cross-section for the opening. The inner diameter of the drawingpipe is 2 mm.

The method of drawing the water and the measurement of the fineparticles are the same as during for the background measurementdescribed above. Here as well, it is important that the measurement beconducted in a stable state, without generation of air bubbles. A totalof 30 continuous measurements are taken, and since the first 10 do notgive stable fine particle concentrations due to air bubbles, etc., the20 measurements from the 11th to the 30th measurement are treated as themeasurement data, and the average thereof is calculated.

The measurement results gave an average of 24.5 per 10 ml, with standarddeviation 3.85, for fine particles of 0.3 μm or greater with a testpiece of the surface (outer) of the bag, and an average of 27.3 per 10ml, with standard deviation 4.03, for fine particles with a test pieceof the inner surface of the bag.

The background value was then subtracted therefrom and the value wasconverted relative to 1 ml, which gave a number of fine particles on thesurface of the bag of 2.3 per ml on the outer surface and 2.6 per ml onthe inner surface.

As a comparative example, the ultrasonic vibrator 5 and ultrapure waterspraying nozzle 7 were removed from the apparatus shown in FIG. 2, afterwhich the rest of the apparatus shown in FIG. 1 was used for cleaning ofa plastic bag in the same manner as the example, and the concentrationof fine particles was measured.

As a result, the concentration (number) of fine particles on the surfaceof the bag was 30.2 per ml, with a standard deviation of 8.26.

It has not been a conventional practice to perform high cleaning ofplastic films using ultrapure water in a clean room. The cleanliness ofconventional plastic films and sheets has been on the order of, forexample, an amount of 10,000 per ml of particles with a size of 0.3 μmor greater.

The bag used above was prepared after cleaning of a tube-shaped plasticfilm, but it will be obvious that the cleaning may also be performedsimultaneously on both sides of a single plastic film or sheet.

Industrial Applicability

According to the present invention, both sides of plastic films andsheets, or the insides and outsides of plastic bags, may be cleaned to ahigh degree not possible according to the prior art. Such highly cleanedplastic films and sheets, and especially bags made therefrom, aresuitable for use in the field of semiconductors, etc. which requireespecially high degrees of cleanliness.

Furthermore, the process for producing them is highly productive as itmay be carried out continuously and in a highly efficient manner, fromthe formation of the plastic film or sheet to the washing with ultrapurewater.

What is claimed is:
 1. A highly clean plastic film or sheet, whereinupon immersing a test piece of the plastic film or sheet in ultrapurewater, extracting pure water from a vicinity of the surface of said testpiece and measuring the concentration (number) of fine particles 0.3 μmor greater in size dispersed in the extracted ultrapure water, theconcentration is no greater than 1000 per ml on both sides of theplastic film or sheet.
 2. A highly clean plastic film or sheet accordingto claim 1, wherein said concentration is no greater than 10 per ml. 3.A highly clean plastic film or sheet according to claim 1, wherein saidconcentration is no greater than 5 per ml.
 4. A highly clean plasticfilm or sheet according to claim 3, wherein the surface resistance ofsaid plastic film or sheet is no greater than 10⁹ Ω/□.
 5. A highly cleanplastic bag which is constructed from a highly clean plastic film orsheet according to claim
 1. 6. A highly clean plastic bag according toclaim 5, wherein said plastic film or sheet has a thickness of 30-150μm.
 7. A process for high cleaning of a plastic film or sheet, whereinthe high-cleaning process for the plastic film or sheet involves(A)highly cleaning the plastic film or sheet in a clean room, and (B) saidhigh-cleaning process includes the steps ofi) immersing the plastic filmor sheet in ultrapure water for washing, ii) exposing both sides of theplastic film or sheet drawn from said ultrapure water to spraying withultrapure water for forcible washing, iii) removing the water from theplastic film or sheet, and iv) destaticizing the plastic film or sheet.8. The process according to claim 7, wherein said clean room has acleanliness of class 100 or greater.
 9. The process according to claim7, wherein said clean room has a cleanliness of class 10 or greater. 10.The process according to claim 7, wherein ultrasonic waves are appliedto said ultrapure water in which said plastic film or sheet is immersed.11. The process according to claim 7, wherein said forcible washing isaccomplished by spraying of ultrapure water.
 12. The process accordingto claim 7, wherein said water removal is accomplished by blowing itwith hot air using an air knife.
 13. The process according to claim 7,wherein said high cleaning comprises the steps of immersing and runninga continuous plastic film or sheet in ultrapure water, exposing bothsides of the plastic film or sheet drawn from the ultrapure water tospraying with ultrapure water for forcible washing, removing the waterfrom the plastic sheet by blowing it with hot air with an air knife, andthen subjecting the plastic film or sheet to voltage application-typeelectrostatic destaticization treatment, all in a clean room of class 10or greater.
 14. The process according to claim 7, wherein said plasticfilm or sheet is tube-shaped, and a step is included of forming a bagfrom the plastic film or sheet by cutting and/or heat sealing in saidclean room, after high cleaning treatment of said tube-shaped plasticfilm or sheet.
 15. The process according to claim 14, wherein saidtube-shaped plastic film or sheet is a tube-shaped film or sheetobtained by an inflation film molding method using high-purity gas. 16.The process according to claim 7, wherein a plastic film or sheet with asurface electric resistance of 10⁹ Ω/□ or less is used as the startingplastic film or sheet.
 17. The process according to claim 7, whereinupon immersing a test piece of the plastic film or sheet in ultrapurewater, extracting pure water from near the surface of said test pieceand measuring the concentration (number) of fine particles 0.3 μm orgreater in size dispersed in the extracted ultrapure water, theconcentration of said plastic film or sheet obtained by said highcleaning is no greater than 10 per ml on both sides of the plastic filmor sheet.
 18. The process according to claim 17, wherein saidconcentration is no greater than 5 per ml.